Method for communicating with RF transponders

ABSTRACT

A method of selecting groups of radio frequency RF transponders (tags) for communication between a base station and the tags. The tags are selected into groups according to a physical attribute of the signal sent by the tags to the base station, or according to the physical response of the tags to a physical attribute of the signal sent from the base station to the tags. Communication with the tags is thereby simplified, and the time taken to communicate with the first tag is markedly reduced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/720,598,filed Sep. 30, 1996, now U.S. Pat. No. 5,777,561, issued Jul. 7, 1998.

FIELD OF THE INVENTION

The field of the invention is the field of Radio Frequency (RF)Transponders (RF Tags), wherein a Base Station sends power andinformation to one or more RF Tags which contain logic and memorycircuits for storing information about objects, people, items, oranimals associated with the RF Tags. The RF Tags can be used foridentification and location (RID Tags) of objects and to sendinformation to the base station by modulating the load on an RF Tagantenna.

BACKGROUND OF THE INVENTION

RF Tags can be used in a multiplicity of ways for locating andidentifying accompanying objects, items, animals, and people, whetherthese objects, items, animals, and people are stationary or mobile, andtransmitting information about the state of the of the objects, items,animals, and people. It has been known since the early 60's in U.S. Pat.No. 3,098,971 by R. M. Richardson, that electronic components on atransponder could be powered by radio frequency (RF) power sent by a"base station" at a carrier frequency and received by an antenna on thetag. The signal picked up by the tag antenna induces an alternatingcurrent in the antenna which can be rectified by an RF diode and therectified current can be used for a power supply for the electroniccomponents. The tag antenna loading is changed by something that was tobe measured, for example a microphone resistance in the cited patent.The oscillating current induced in the tag antenna from the incoming RFenergy would thus be changed, and the change in the oscillating currentled to a change in the RF power radiated from the tag antenna. Thischange in the radiated power from the tag antenna be picked up by thebase station antenna and thus the microphone would in effect broadcastpower without itself having a self contained power supply. In the citedpatent, the antenna current also oscillates at a harmonic of the carrierfrequency because the diode current contains a doubled frequencycomponent, and this frequency can be picked up and sorted out from thecarrier frequency much more easily than if it were merely reflected.Since this type of tag carries no power supply of its own, it is calleda "passive" tag to distinguish it from an active tag containing abattery. The battery supplies energy to run the active tag electronics,but not to broadcast the information from the tag antenna. An active tagalso changes the loading on the tag antenna for the purpose oftransmitting information to the base station.

The "rebroadcast" of the incoming RF energy at the carrier frequency isconventionally called "back scattering", even though the tag broadcaststhe energy in a pattern determined solely by the tag antenna and most ofthe energy may not be directed "back" to the transmitting antenna.

In the 70's, suggestions to use tags with logic and read/write memorieswere made. In this way, the tag could not only be used to measure somecharacteristic, for example the temperature of an animal in U.S. Pat.No. 4,075,632 to Baldwin et. al., but could also identify the animal.The antenna load was changed by use of a transistor.

Prior art tags have used electronic logic and memory circuits andreceiver circuits and modulator circuits for receiving information fromthe base station and for sending information from the tag to the basestation.

The continuing march of semiconductor technology to smaller, faster, andless power hungry has allowed enormous increases of function andenormous drop of cost of such tags. Presently available research anddevelopment technology will also allow new function and differentproducts in communications technology.

U.S. Pat. No. 5,214,410, hereby incorporated by reference, teaches amethod for a base station to communicate with a plurality of Tags . Thetags having a particular code are energized, and send a response signalat random times. If the base station can read a tag unimpeded by signalsfrom other tags, the base station interrupts the interrogation signal,and the tag which is sending and has been identified shuts down. Theprocess continues until all tags in the field have been identified. Ifthe number of possible tags in the field is large, this process can takea very long time. The average time between the random responses of thetags must be set very long so that there is a reasonable probabilitythat a tag can communicate in a time window free of interference fromthe other tags.

RELATED APPLICATIONS

Copending patent applications assigned to the assignee of the presentinvention and hereby incorporated by reference, are:

Ser. No. 08/303,965, filed Sep. 9, 1994 entitled RF Group SelectProtocol, by Cesar et al, now U.S. Pat. No. 5,670,037;

Ser. No. 08/304,340, filed Sep. 9, 1994 entitled Multiple Item RF IDprotocol, by Chan et al, now U.S. Pat. No. 5,550,547;

Ser. No. 08/521,898, filed Aug. 31, 1995 entitled Diode Modulator for RFTransponder by Friedman et al, now U.S. Pat. No. 5,606,323;

application submitted Aug. 9, 1996, entitled RFID System with BroadcastCapability by Cesar et al; and

application submitted Jul. 29, 1996 entitled RFID transponder withElectronic Circuitry Enabling and Disabling Capability, by Heinrich etal.

These applications teach a communications protocol whereby a basestation communicates to a plurality of tags by polling the tags andshutting down tags in turn until there is just one left The informationis then exchanged between the base station and the one tag, and then theone tag is turned off. The unidentified tags are then turned on, and theprocess is repeated until all the tags have the communication protocolcompleted. Typical protocols requires a time which is not linearlyproportional to the number of tags in the field More tags take a longertime per tag than fewer tags. If the tags can be selected into groups insome way, each group can be dealt with in a shorter time per tag, andthe time taken to communicate with the first tag is markedly shortened.

SUMMARY OF THE INVENTION

The method of the present invention is a method of selecting groups ofRF tags for a communication protocol comprising selecting a plurality ofgroups of tags according to a physical attribute of the signal sent bythe tags to the base station, or selecting the groups according to thephysical response of the tags to a physical attribute of the signal sentfrom the base station to the tags, and communicating with the tags ineach group. A single tag may be a member of one or more groups. Somegroups may have no members. The most preferred embodiment of theinvention is the method of selecting groups on the basis of the physicalsignal strength of the RF signal received from the tags by the basestation. The tags have greater or less received signal strengthdepending on the distance to the base station antenna, the relativeorientation of the tag and the base station antennas, and the localconditions of reflectors and absorbers of radiation around the tag. Thebase station may also select groups of tags according to thepolarization or the phase of the returned RF signal, the RF carrier orDoppler shifted RF carrier or modulation frequency sent by the tags, orany another physical signal from the tags. The base station may alsoselect groups of tags according to the physical response of the tags tothe polarization, phase, carrier frequency, modulation frequency, orpower of the RF signal sent by the base station. The communicationprotocol can be carried out simultaneously or sequentially with theselected groups. The physical characteristics used to group the tags canbe measured simultaneously or sequentially. Different groups may beselected by taking the union, the intersection, or other combinations ofthe various groups of tags selected according to the different physicalattributes. The tag group selection parameters may also includeselecting groups by software, i.e. by selecting the groups according toinformation stored on the tag.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized diagram of a base station communicating to oneor more tags.

FIG. 2 is a diagram of a base station having two antennas for receivinginformation about the polarization of the signal sent by a tag.

FIG. 3 is a diagram of a base station having three antennas forreceiving information about the polarization and phase position of thesignal sent by a tag.

FIG. 4 is a diagram of a base station circuit which can select thestrongest signals from signals sent by a plurality of tags.

FIG. 5 is a flow chart of the most preferred embodiment of theinvention.

FIG. 6 is a flow chart of a preferred embodiment of the invention.

FIG. 7 is a flow chart of a preferred embodiment of the invention.

FIG. 8 is a flow chart of a preferred embodiment of the invention.

FIG. 9 is a flow chart of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 sketches abase station 10 sending RF energy 21 and information toone or more tags 20. The tags 20 may have varying distances from thebase station, and the tag antennas 22 may be in any orientation withrespect to the base station antenna. The base station comprises atransmitter section 100, a computer section 50, a circulator 170, areceiver section 200, and one or more antennas 185.

FIG. 2 depicts a base station 10 which can group the tags 20 into groupson the basis of polarization of the RF radiation back scattered to thebase station 10. The base station 10 has two perpendicular antennas 185and 185' communicating with three tags 20, 20', and 20". The antennas185 and 185', and 22, 22' and 22" are depicted as simple dipole antennaswhich transmit linearly polarized radiation with the polarizationsubstantially parallel to the antennas. In the diagram shown, antenna185 may communicate well with the tag 20 having an antenna 22 parallelto antenna 185, less well with the antenna 22" which is shown having a45 degree orientation with respect to antenna 185, and not at all withthe tag with a perpendicular antenna 22'. The groups are first selectedon the a basis of the response of the tags to the polarization of thesignal sent out from the base station. In this example, two groups areselected: those tags which respond to the particular polarization, andthose tags which do not respond. In the embodiment depicted in FIG. 2, asignal sent out from antenna 185 brings responses from tag 20 and fromtag 20" to antenna 185, and from tag 20" alone to antenna 185'. The tagantenna 22' may not receive power from the perpendicular antenna 185,and so tag 20' remains silent. The tags are then further selected intosubgroups according to the polarization of the returned signal. Thus,three groups of tags are selected by this method in this example, tag20' is in one group of "silent" tags, tag 20" is in the group which ispicked up by antenna 185' because the polarization of the signal fromtag 20" can be detected by antenna 185', and tags 20 and 20" are in thegroup with polarization components which may be picked up by antenna185. Communication with each of the two "non silent" groups in turn orin parallel simplifies and speed the communication protocol. Inparticular, the time taken to communicate with the first tag is markedlyreduced. In the example given above, the signal returned to antenna 185'is the signal from only a single tag 20", and that tag can return thetag identification number while the antenna 185 receives signalsignifying more than one tag in the field. The tag 20" may then beturned off for the duration of the communication procedure, and theprocess repeated to identify and shut down tag 20. The sending antennais then switched to antenna 185', and the remaining tag 20' isidentified. While a linear polarization scheme is shown as an example,it is clear to one skilled in the art that circularly polarized signalscould also be used with good effect. The exact orientations of theantennas are also not critical to the invention, as long as there is adifference in the sensitivity of the antennas to the polarization of theRF signals sent by the tags. A single base station antenna could beused, as long as the polarization characteristics of the single basestation antenna could be changed by the base station or by other means.

FIG. 3 shows a base station 10 with more than two dipole antennas 185,185', and 185". In this example, each antenna axis is mutuallyorthogonal so that the orientation of the linearly polarizedbackscattering from dipole antennas 22 in the field can be measured andthe tags selected into groups for the communication procedure.

FIG. 4 shows a block diagram for circuitry which can allow the basestation to select a group of tags by the signal strength received at thebase station. The equipment for implementing the method of the mostpreferred embodiment of the invention uses five sections of the basestation 10: a computer section 50, a transmitter section 100; a receiversection 200; a hybrid coupling device 170; and an antenna 185. Thecomputer section may be a relatively unsophisticated circuit forcontrolling the transmitter and for receiving signals from the receiver,or it could include highly sophisticated workstations for interrogatingand writing information to the tags. The transmitter section 100, undercontrol of the computer section 50, sends a signal of the appropriateamplitude and frequency (which may or may not be modulated) to thehybrid 170, which sends the (modulated) signal to the antenna 185. Thepreferred modulation for communication to and from the tags is amplitudemodulation, but it may be either frequency or phase modulation. Theantenna 185 both sends out the RF carrier frequency which may or may notbe modulated, and captures the signals radiated by the tags 20. Theantenna 185 captures the signals radiated by the tags and sends thesignals back to the hybrid 170, which sends the signals to the receiversection 200. The receiver section down converts and extracts themodulated signal from the carrier, and converts all the modulationenergy it receives to a baseband information signal at its output. Inthe most preferred embodiment, the receiver has two outputs inquadrature called I (in phase with the transmitted carrier) and Q(quadrature, 90 degrees out of phase with the carrier). However, variousembodiments of the invention have just one output. The hybrid element170 connects the transmitter and receiver to an antenna whilesimultaneously isolating the transmitter and the receiver from eachother. That is, the hybrid allows the antenna to send out a strongsignal from the transmitter while simultaneously receiving a weakbackscattered reflection. The strong transmitted signals being sent intothe antenna must be eliminated from the receiver by the hybrid.

The transmitter section depicted by block 100 provides the energy andfrequency signals for the transmitter carrier and the receiver downconverter, and the amplified and modulated signal 160 which may be sentby the antenna 185. The RF source 105 of signal 110 is usually isolatedby an element 120 between the carrier signal source 105 and the rest ofthe circuit which avoids coupling problems of coupling reflections backto the RF source. The isolation element 120 is usually a circulator withone port terminated by a resistor. The isolated carrier signal 125 issplit into two paths in a signal splitter element 130. Most of theenergy 140 goes to an amplifier modulator element 150, while signal 135takes a small signal to the receiver section depicted by block 200. Anoptional phase and/or frequency shifter element 139 may be includedbetween the signal splitter 130 and the receiver section 200 to providecontrol by the computer section 50 over line 157 of the reference phaseand frequency signal 210 which the receiver section uses in detectingthe signals from the tags. The phase and or frequency shifter 139 maysend out signals differing by a small amount in frequency from thesignal 110 sent out from the RF source 105, or it may send out harmonicsof the signal. In the amplifier modulator section 150, the carrierfrequency is amplified and modulated by a signal 155 controlled bycomputer section 50. A preferred embodiment has a carrier frequencygreater than 400 MHZ. A more preferred embodiment has a carrierfrequency greater than 900 MHZ. The most preferred embodiment uses acarrier frequency of from 2.3 to 2.5 Ghz, and this signal is amplitudemodulated at 20-60 kHz. In the preferred embodiment, a direct modulationof the carrier frequency is depicted However, an up converter ofmultiple frequencies may also be used. This modulated signal 160 entersthe hybrid element 170 and is passed over lead 180 to the antenna 185. Amodulator signal is applied at 155 into the modulator 150 to give amodulation which may be amplitude, frequency or phase modulation. Themost preferred embodiment is amplitude modulation.

In the receiver section 200, the received signal from the antenna 185travels along lead 180 and enters the hybrid 170 which directs thesignal along 220 to the receiver section depicted by block 200. Thissignal comprises signals sent by the tags, which modulate the carrierfrequency at a frequency of; for example, 40 KHz, and the reflectedunmodulated transmitter carrier signal reflected from the antennas orother reflectors in the field. The antenna will never be perfectlymatched to the transmitter, and will reflect a signal which is about 20dB down from the signal transmitted by the antenna. Of course, thecarrier signals reflected by the tags, and the various reflections ofthe transmitted signal, will be much weaker than the signal transmittedfrom the antenna. The receiver structure 230 of the most preferredembodiment here is a direct down conversion I and Q system where themixing frequency signal 210 is generated by the source 105 and is theonly send-out by the transmitter. The single down conversion systemreceiver removes the carrier frequency signal and generates two basebandsignals which have frequencies in the 40 KHz region in quadrature 310and 410. These signals are filtered and amplified by means of signalprocessing in elements 300 and 400. The signals 320 and 420 are passedto the computer section 50 for further processing.

The hybrid component 170 is typically a circulator. It passes signalsfrom 160 to 180, from 180 to 220, from 220 to 160 but not the other wayaround. Hence the transmitter is isolated from both the small amount ofmodulated carrier reflected by the antenna 185 (20 dB down typically)and the circulator (20 dB leakage typically). The receiver is isolatedfrom the large signal sent from the transmitter 100 to the antenna 185,and receives about -20 dB signal from leakage from the circulator 170and a further -20 dB of signal from the reflection from the antenna.

Of course, when the base station modulates the carrier signal totransfer information from the base station to the tags, the reflectedmodulated signals from the antenna and the leakage from the circulatorwill swamp out any signals sent by the tags. In the prior art the tagscommunicate in a time period when there is no modulation of the carriersignal transmitted from the base station, or the tags communicate at adifferent carrier frequency than that transmitted by the base station,so that the receiver can pick out the modulated signals from the tagsfrom all the reflections and leakages of the carrier signals. Thepresent invention allows simple discrimination of signals by the tag tothe base station sent as modulation of the base station carrierfrequency, or as modulations of another frequency, from one or moretags, and allows the tags to be sorted in groups determined by the tagsignal strength received at the base station.

The most preferred embodiment of the present invention is a method tosort the tags into groups by sending a steady, weak signal modulation atthe communication modulation frequency to the tags in the time periodwhere the prior art sends an unmodulated carrier signal so that the tagsmay communicate back to the base station. The steady, weak modulationfrequency is not strong enough to influence the tag, but is strongenough so that the steady, weak modulated signals reflected from theantenna 185 and leaked around the hybrid 170 can be measured by thereceiver and can be used to set a level for discriminating amongst thetag signals. In the most preferred embodiment, the communication to thetags is carried out by a 100% amplitude modulation of the carrierfrequency at a 20-60 Khz frequency. The preferred protocol for the tagsto detect such information is a 50 dB on/off ratio, but this is notnecessary to the invention. Any modulation of the carrier frequencywhich can conceivably be used for communication between the tags and thebase station can be used. Such modulations as frequency modulation andphase modulation are well known in the art. In the present invention, amodulation amplitude less than that used to communicate with the tags isimpressed on the outgoing carrier wave. The mismatch at the antenna willalways cause that signal to be reflected and to be present at thereceiver. This signal is detected at the receiver and is used toestablish a deterministic signal floor. As backscattered modulatedsignals are received and are stronger than this coupling signal, thereceived back scattered signal dominates the receiver. Hence, a highsensitivity receiver may be used with a forced coupled modulation fromthe transmitter as its signal noise floor, and behave in a predictablemanner between the conditions of no tags in the field, a single tag inthe field, multiple tags in the field, and interference. Furthermore, byvarying the modulation strength of the weak, modulated signal, thereturned signal strength of signals from the tags required to overcomethe coupled modulator signal is increased or decreased thereby allowingthe base station to select a group of tags based on the returned signalstrength.

FIG. 5 depicts a flow chart 500 of the most preferred method forselecting groups of tags and communicating with the tags in each group.A modulation frequency of 40 Khz is chosen as an example. At step 510,the base station transmits a modulated signal to the base stationantenna, and hence to the tags, instructing the tags to respond andreturn a modulated signal in a time period (time slot) defined by thetag communication protocol. At step 520, the base station transmits acarrier wave to the base station antenna The carrier wave has a steady40 Khz amplitude modulation which is less than that required tocommunicate with the tags. The base station measures the 40 KHZmodulation received from the base station antenna in the time slotdefined by the tag communication protocol. If the modulated signalreceived by the receiver 200 is steady in step 530, the reflectedmodulated signal and leakage is greater than any signals received fromtags, which would send an unsteady modulated signal. The base stationthen reduces the amplitude of the steady modulated signal in step 540and the system returns to step 510. If the modulated signal is notsteady in step 530, the base station checks at step 550 to see whetherthe modulated signal returned is steady outside the time slot defined bythe tag communication protocol. If the modulated signal is unsteady whenno tags are supposed to be sending signals, the unsteady signal isnoise, and the receiver can not distinguish between signals sent by thetags and the noise. No tags are in reading position in the field, andthe protocol is ended in step 560. If however the modulated signal issteady outside the time slot, and unsteady in the time slot, one or moretags in the field are sending signals. These signals are stronger thanthe steady modulated signals received from the reflected steadilymodulated carrier wave. If a single tag is in the field, and can be readat step 570, the single tag is read and instructed to shut off, at step590, and the system is returned to step 540 to reduce the steadymodulation and return to the beginning step 510 to try to find tags withless signal strength. If more than one tag is in the field and the tagsignals interfere with each other so that they can not be read at step570, a multiple tag reading protocol is instituted in order to read themultiple tags at step 580. The tags are read using the multiple tagreading protocol, and ordered to shut down, and the system is returnedto step 540 to reduce the steady modulation and return to the beginningstep 510 to try to find the group of tags with less signal strength thanthe first group.

Step 550 is preferably taken after step 530, but step 550 may optionallybe taken between steps 570 and 580 or after step 580 if no tags are readby the multiple tag reading procedure.

The most preferred embodiment of the invention uses a protocol in whichthe tags are commanded to return an identification signal in aparticular time slot, but the same invention may be used where the tagsare commanded to return information in any defined time periods.

While the preferred embodiment uses the naturally occurring reflectionsfrom the base station antenna 185 and leakage from the hybrid 170 tointroduce the noise floor signal into the receiver 200, many other meansof introducing this signal to the receiver are possible to one skilledin the art. As an example, the steady 40 Khz modulation could be summedwith the signals from the I/Q demodulator coming on lines 310 and 410,or indeed a specially constructed device analogous to a two input I/Qdemodulator could be constructed to accept the steady 40 Khz comparisonsignal from an outside source.

Additional embodiments of the invention include further subdividing thegroups selected by the above method on the basis of the phase and/orpolarization of the signals returned to the base station, as well asother physical or software group selection criteria.

A preferred embodiment of the invention is to select tags on the basisof the returned polarization of the signals. In the embodiment shown inFIG. 2, groups of tags with antennas which return a linear polarizationwhich is polarized more parallel to one or the other of the two dipoleantennas 185 or 185' sketched in FIG. 2 are selected. Returned signalsfrom the two antennas are processed in parallel by two sets of receivercircuitry like that shown in FIG. 4. The tags are interrogated bytransmitting the modulated carrier signal from first one antenna 185,then the other antenna 185', and four channels of signals (the I and Qchannels received from each antenna) may be processed in parallel or insequential fashion. This set up would select the tags into 8 groups,which of course may be further selected and grouped on the basis of thereceived signal strength or any other physical or software attribute.

FIG. 6 depicts a flow chart 600 of the preferred method of selectinggroups of tags on the basis of the polarization of the signals returnedto the base station. As an illustrative example, a base stationcomprising 2 antennas which are sensitive to different polarizations,such as depicted in FIG. 2, is chosen. However, the number of antennasand whether the polarization is linear, circular, or some combination ofthe polarizations may be chosen at will by one skilled in the art. Step610 uses antenna 185 to send a signal to the tags instructing the tagsto return a signal in the time slot determined by the communicationprotocol. The antenna 185 is then used to listen for signals from thetags in the time slot where the tags return signals in step 620. Signalsretuning from antenna 185 are analysed in step 630 to see if the basestation can read the signal. If the signal is returned from a singletag, the base station communicates with the single tag in step 640, andinstructs the tag to shut itself down for the remainder of thecommunication protocol, or until it is specifically instructed to startreturning signals again. The system is then returned to step 610 to lookfor more tags. If the signal returned by the tags to antenna 185 can notbe read, either because there are no tags in the field in a position tobe read by antenna 185 or because there multiple tags trying tocommunicate at the same time, the system may then try to read a singletag communicating to antenna 185' in step 650. If a single tag issuccessfully read, the system reads the tag at step 640, shuts the tagdown, and returns to the beginning step 610 to try to read again thetags which may be tying to communicate to antenna 185. Since there isnow one fewer tag in the field, a tag may now be read at step 630 onantenna 185. If a single tag can not be read in step 650, a multiple tagin the field reading procedure is instituted in step 660. Steps 630 and650 may be taken either sequentially or simultaneously, if two receiversare connected to the two antennas. If tags are read using one antenna instep 660, the system decides in step 670 to communicate with the tagsand turn them off and the system returns to step 610 to try to read asingle or multiple tag from the other antenna If the multiple tagreading procedure does not read any tags from either antenna in stop660, the system may switch transmitting antennas in step 680, so thatthe commands and carrier wave are transmitted to antenna 185' instead ofantenna 185. The method 600 of the invention can then be used toidentify and select other groups not found in the first application ofmethod 600. Alternatively, the system may switch transmitting antennasbetween steps 650 and 660 to try to find, communicate with, and shut offsingle tags.

Another antenna perpendicular to the two antennas shown in FIG. 2, whichis placed remotely from the base station as shown in FIG. 3 allows allcombinations of linear polarized backscattering to be discriminated andallows the selecting of groups based on all polarizations of thereceived signal.

The three antennas 185, 185', and 185" shown in FIG. 3 allow many moregroups to be selected on the basis of phase information. A possiblydifferent group responds in the I and Q channels of the receiver of eachantenna, and the groups may be different depending on which antenna orcombination of antennas sends the carrier signal to the tags. Such groupselection markedly cuts down the time needed to interrogate many tags inthe field.

Base station antennas and tag antennas sensitive to circular and otherpolarizations are also known in the art, and these also may be used byone skilled in the art in an analogous way to that shown in FIGS. 1, 2,and 3 and described above.

An additional preferred embodiment of the invention is to use theinformation on the I and Q channels to select tags into groups on thebasis of the phase of the returned signal which is dependent on thedistance of the tags from the base station. As a tag is moved away fromthe base station, the carrier signal from the tag received at the basestation changes from being in phase with the transmitted signal to being90 degrees out of phase to being 180 degrees out of phase as the tag ismoved one quarter of a wavelength of the RF EM field. The amplitude inthe I channel and the Q channel changes accordingly, for example from a1 in the I channel and a 0 in the Q channel, to a 0 in the I channel anda 1 in the Q channel, to a -1 in the I channel and 0 in the Q channelrespectively. Thus, selecting the signals received from the tags on theI channel alone selects a group of tags for communication, whileselecting the signals received from the tags on the Q channel selects adifferent group of tags which are at different distances from the basestation antenna. Both the I and the Q channels may be usedsimultaneously or sequentially to communicate with the two differentgroups of tags. It is possible that some tags may be in both groups atthe same time. As long as there are some tags in one group and not inthe other, the selecting of the groups speeds up the tag communicationprotocol.

FIG. 7 gives a flow chart of a preferred method 700 of selecting groupsof tags by the phase of the signal returned to the base station. Asignal 710 is sent from the base station to the tags instructing thetags to return modulated signals to the base station in the time slotdesignated for tag response. In this time period, a steady carrier wavehaving a defined phase is transmitted 720 from the base station antenna.If a single tag can be read on the receiver I channel 730, the tag isinstructed to shut itself off in step 740 and the system returns to step710. If a single tag can not be read on the I channel in step 730, thesystem tries to read a single tag in the Q channel in step 750. If asingle tag can be read step 750, the tag is instructed to shut itselfoff in step 740, and the system returns to the beginning 710 to try topick up a single tag in the I channel. If single tags can not be read ineither the I channel or the Q channel, the system decides in step 750 toinstitute the multiple tag in field reading procedure 760. If tags areidentified in either I or Q channels in step 760, the system may shutthe identified tags off and return to step 710 to try to find singletags grouped in the other channel.

While the above method 700 has steps 730 and 750 proceedingsequentially, it is well within the scope of the invention that steps730 and 750 may also be carried out simultaneously. If a single tag isread on either the I channel or the Q channel, the system returns tostep 710. If no single tags are read on steps 730 and 750, the systemproceeds to step 760. In step 760, if tags are identified and shut off,the system may at any time return to step 710 to carry out the simplersubgrouping.

With the addition of an optional phase shifting element 139, signalsfrom a particular tag are brought entirely into the I channel or the Qchannel. The tags may then be sorted into many more groups than the twogroups defined by the I and Q channels as explained above. If only onechannel of information, for example the I channel, is used, changing thephase shifting element 139 to give a series of different phase delaysmay sort the tags into more groups. The computer section 50 may end thephase shift element 135 instructions over line 157 to shift phase by,for example 0, 30, 60, and 90 degrees which would select four differentgroups of tags for communication. Using both the I and Q channels, and 3phase shifts of 0, 30, and 60 degrees gives 6 groups as another example.

If the carrier signal frequency sent out from the base station ischanged, a particular tag will be a different number of quarterwavelengths from the base station and the signal will be distributed ina different way between the I and Q channels of the base stationreceiver. A preferred embodiment of the present invention is to selectdifferent groups of tags according to the response of the tag to such afrequency shift of the base station. FIG. 8 gives a flow chart for themethod 800 of selecting groups of tags on the basis of the response ofthe tag to the frequency of the carrier signal sent out from the basestation. In step 810, the base station sends out a carrier wave having afirst frequency f₁. In step 820, the base station instructs the tags toreturn signals. The signal returning to the base station is analyzed ina single channel of the receiver in step 830. If the signal can be read,the tag is communicated with and turned off in step 840 and the systemreturns to step 820 to find single tags which may have less receivedsignal strength than the tag found in the previous cycle. If no tag isfound in step 830, the system then changes the carrier frequency sentout from the base station in step 850 to a frequency f₂, and then sendssignals to the tags to return signals in step 860. If a single tag canbe read in step 870, the tag is communicated with and shut off in step880, and the system returned to step 860. If no tags are found in step870, the system checks to see if any tags have been found in previouscycles through step 870, and if so the system is returned to thebeginning step 810 to search the first frequency again If no tags havebeen found in previous cycles, the system goes to the multiple tag inthe field search procedure 890. While two frequencies are used in thisexample, the method is not limited to the use of just two frequencies,and many more could be used. Use of any plurality of frequencies whichshift the relative phase of the returned signal is contemplated by theinventors.

A further embodiment of the invention is to select the tags into groupson the basis of the frequency response of the tags. Tags responsive todifferent carrier frequencies are interrogated, and the base station isprogrammed to shift from one frequency to the next to select andinterrogate these different groups of tags in a sequential fashion. Tagsmay be grouped into tags which respond to 900 MHZ, and tags whichrespond to 2.4 MHZ, as an example.

A further embodiment of the invention is to select the tags into groupson the basis of the response of the tags to the RF power transmittedfrom the base station. The method of the embodiment is to send a lowpower to the set of tags, and communicate with the set of tags whichrespond to the low power, then turn the tags which responded to the lowpower off. Next, the RF power transmitted from the base station israised, and tags in a group which are further away than the first grouprespond, and are in turn communicated with and turned off. The processmay be repeated until all tags in communication range of the basestation with the maximum power allowed have finished the communicationprotocol.

Tags which themselves return different carrier frequencies than the basestation carrier frequency are known in the art. A further embodiment ofthe invention is to select groups of such tags on the basis of thedifferent measured carrier frequencies. The base station is programmedto receive the different tag carrier frequencies, either simultaneouslyor sequentially and to interrogate each group of tags. The differentcarrier frequencies known in the art are often the harmonics of the basestation carrier frequency. However, the invention is not limited to theparticular carrier frequency returned by the tags to the base station.If the tags can be selected into at least two groups, the communicationprotocol is speeded up.

FIG. 9 is a flow chart of a method of grouping the tags on the basis ofthe carrier frequency of the tags. The receiver is set to receive acarrier signal of frequency f₁ in step 910. Step 920 instructs the tagsto return signals. If a single tag is read in step 930, the systeminstructs the tag in step 940 to turn off and return to step 920. If notag can be read in step 930, the receiver frequency is changed in step950 to f₂, and the tags are instructed in step 960 to return signals. Ifa single tag can be read in step 970, the tag is communicated with andshut off in step 980. If a single tag can not be read in step 970, themultiple tag reading protocol is instituted. While two frequencies areused in this example, many more frequencies could also be used.

The carrier frequencies emitted by the tags and received by the basestation may be apparently shifted from the base station carrierfrequency by the Doppler shift due to the relative motion of the tagsand the base station A further embodiment of the invention is to selectgroups of tags according to the Doppler shift of the carrier frequencysent by the tags and received by the base station. As an example, twogroups of tags, those with relative motion of the tags towards the basestation, and those with relative motion away from the base station, areselected for the communication protocol. This group selection isparticularly valuable for a base station communicating with tags on oneside of a doorway, for example, to measure whether the tags are carriedinto or out of a room.

Tags may return different modulation frequencies. A further embodimentof the invention is to select groups of tags on the basis of themodulation frequency of the returned tag signal. The base station isprogrammed to interrogate each group of tags either simultaneously orsequentially.

The invention is not limited to the above examples. The selection ofgroups of tags from a set of tags on the basis of any physicallymeasured characteristics or attributes of the returned signal from thetags in response to any physical characteristic or attribute of thesignal sent from the base station is well within the scope of theinvention, as is the combination of the selection of groups on the basisof both physically measured characteristics and information contained onthe tags.

We claim:
 1. A method for communicating between a base station and a setof radio frequency RF transponders (Tags) comprising:defining aplurality of RF tags into different groups according to a physical wavecharacteristic of the electromagnetic wave energy received from the RFtags, and communicating with the tags in each defined group.
 2. A methodas in claim 1 wherein at least one defining wave characteristic is thewave amplitude.
 3. The method of claim 1 wherein at least one definingphysical wave characteristic is the wave frequency.
 4. The method ofclaim 1 wherein at least one defining physical wave characteristic isthe polarization of the signal.
 5. The method of claim 1 wherein atleast one defining physical wave characteristic is the phase shift of areturn signal.
 6. The method of claim 1 wherein at least one definingphysical wave characteristic is the strength of tie signal.
 7. Themethod of claim 1 wherein at least one defining physical wavecharacteristic is the amplitude modulation of the signal.
 8. The methodof claim 1 wherein at least one defining physical wave characteristic isthe wavelength of the signal.
 9. An RF tag base station comprisingacomputer a transmitter a receiver, and at least one antenna, wherein theRF tag base station communicates with a plurality of RF tagsby:interrogating the RF tags with electromagnetic energy, grouping theRF tags according to a physical characteristic of their responsiveelectromagnetic signals, and reading the RF tags in each group.
 10. Abase station as in claim 9 wherein RF tags are grouped according to thewave amplitudes of their respective return signals.
 11. A base stationas in claim 9 wherein RF tags are grouped according to the wavefrequency of their respective return signals.
 12. A base station as inclaim 9 wherein RF tags are grouped according to the polarization oftheir respective return signals.
 13. A base station as in claim 9wherein RF tags are grouped according to the phase shift of theirrespective return signals.
 14. A base station as in claim 9 wherein RFtags are grouped according to the strength of their respective returnsignals.
 15. A base station as in claim 9 wherein RF tags are groupedaccording to the amplitude modulation of their respective returnsignals.
 16. A base station as in claim 9 wherein RF tags are groupedaccording to the frequency modulation of their respective returnsignals.
 17. A base station as in claim 9 wherein RF tags are groupedaccording to the wavelength of their respective return signals.
 18. AnRF tag unit reading unit comprising:a computer; a transmitter; areceiver; and at least one antenna; wherein the RF tag reading unitcommunicates with a plurality of RF tags by:interrogating the RF tagswith electromagnetic energy; grouping the RF tags according to aphysical characteristic of their responsive electromagnetic signals, andreading the RF tags in each group.